Electron discharge device



A ril 16, 1935- .1; o. M NALLY ELECTRON DISCHARGE DEVICE Filed March 1, 1935 FIG. 2

ANODE a INVENTOR J.0.Mc NALLV ATTORNEY Patented Apr. 16, 1935 UNITED STATES PATENT GFFICE ELECTRON DISCHARGE DEVICE James 0. McNally, Maplewood, N. J., assignor to Bell Telephone Laboratories, Incorporated, New York, N. Y., a corporation of New York Application Marchi, 1933, Serial No. 659,029

2 Claims.

' single frequency is applied to the control electrode or grid. of an electron discharge device of the amplifier type, the output of the device contains various harmonics of this single frequency as Well as the amplified fundamental frequency. If a complex signal voltage covering a wide frequency band, such as is involved in the translation of speech and music, is applied to the control electrode or grid, it follows that because of the higher order modulation, particularly the second order, the output of the device will contain harmonics of the various fundamental frequencies in the complex signal so that the signal '9 not reproduced with fidelity and exactness.

An object of this invention is to reduce higher order modulation in electron discharge devices and to thereby amplify signal voltages, such as those utilized in the transmission of speech and music, accurately and efiiciently.

In accordance with a feature of this invention, the parameters involved in the structure of an electron discharge device are so correlated that the magnitude of the harmonics, particularly the .econd harmonic, in the output of the device is small in comparison with the magnitude of the translated fundamental frequency.

More specifically, in accordance with this invention, a grid, such as the control electrode, And an anode in an electron discharge device are so shaped and spaced relative to each other that a plurality of electron paths of different impedances are provided'between the cathode and the anode, each path with respect to the other paths having an amplification factor that is lower than that of another path of lower impedance, and a higher amplification factor than a path of higher impedance. 1

The invention and the features thereof will be understood more clearly and fully from the following detailed description with reference to the accompanying drawing in which Fig. l is a perspective View of an electron dis charge device illustrative of one embodiment of this invention, with portions of the enclosing vessel and the anode broken away to show the inner electrodes more clearly; I

Fig. 2 is a diagrammatic end view showing the configuration of the opposed surfaces of the electrodes in the device shown in Fig. 1;

Fig 3 is a sectional view in outline of the electrode surfaces illustrative of another embodiment of this invention;

Fig. 4 illustrates a modification of the electrodes represented in Fig. 3.

Fig. 5 shows a sectional view in outline of the electrode surfaces in another embodiment of this invention in which the control electrode or grid is concave with respect to the cathode.

Fig. 6 shows a sectional view in outline of the electrode surfaces in another embodiment in which the control electrode or grid is convex with respect to the cathode;

Fig. 7 illustrates a modification of the arrangement shown in Fig. 5, in which the anode surface is curved in the same direction as the control electrode or grid but to a lesser degree; and

Fig. 8 illustrates a modification of the arrangement shown in Fig. 6, in which the opposed sur faces of both the anode and the control electrode or grid are convex with respect to the cathode.

It is known that the current in a plate circuit of an electron discharge device may be expressed as a power series in the input voltage on a grid, for example on the control electrode.

Thus

1' ,=a e+a e +a e zi e (l) where i =plate current. I

e=input voltage on a grid or on the control electrode.

a1, a2, a3, etc. are coefficients depending upon the parameters of the tube and the circuit conditions.

By assuming that the contributions to the double frequency term, 1

T cos 2wt,

from thehigher even order powers are sufficiently small to be neglected, it will be seen that the magnitude of the double frequency component I in the plate current will be proportional to as.

It has been ascertained (see Operation of thermionic vacuum tube circuits, F. B. Llewellyn, Bell System Technical Journal, vol. 5, 1926, pp. 433-462) that the coefiicient a; may be expressed as where:

R0 is the plate resistance of the electron discharge device,

Z is the load impedance,

EB is the plate voltage,

E0 is the grid'voltage, and

o is the amplification factor of the device. For practical purposes so that Equation (3) may be rewritten as e 0 M] 1 n 0+Z-) EB m (4) In a three electrode electron discharge device operating at temperature saturation the plate resistance, R0, increases as the plate voltage EB is also positive so that the two terms within the brackets in Equation (4) are cumulative to give a large value of as. To decrease (12, therefore, requires decreasing either The former term is dependent upon the 3/2 power law and, as is known, cannot be decreased appreciably without sacrificing power output. It remains, therefore, in order to decrease a2 and to thereby reduce second order modulation, to decrease the term It would be most desirable, of course, to change the sign of this term and in the limit to equate it to the first term within the brackets in Equation (4), that is to #2 L O H H-z) an;

In accordance with the commonly accepted theory, in three electrode discharge devices, for large negative grid biases, and hence for small plate currents, the electrons constituting the plate current pass through the grid in relatively small streams, the streams passing midway between adjacent grid wires being of the greatest densities. The effect of the charge on the grid upon these streams of greatest densities is relatively small and as a consequence the amplification factor, t, is correspondingly small. As the grid voltage is made less negative, the plate current increases and the increased space charge of the electron streams forces more electrons to pass in the vicinity of the grid wires. The efiect of the charge on the grid upon the electrons in the vicinity of the grid wires is relatively great so that for the conditions described the amplification factor is increased. The effect, therefore, is an increase in the amplification factor with an increase in the plate current, or a decrease in the grid voltage, and may be considered in a sense as one caused by electrons being forced to use the hi her impedance, higher p. portions of the device as the plate current increases.

In order to obtain an appreciable reduction in the increase of u with decreasing negative grid voltage, it is necessary to design the device so that the electrons constituting the plate current will pass through a path having a low 1. at the higher plate currents. In other words the grid should have what may be termed a low a portion through which relatively few electrons will flow at the lower plate currents and the eifect of this low a portion should become prominent as the plate current increases. It follows, therefore, from the foregoing that if the grid or control electrode of an electron discharge device has a plurality of sections one in a path having a low and a high impedance and another in a path having a high ,u and a low impedance, the increasing space charge with increased plate current through the high ,u. section will force a correspondingly larger number of electrons to pass through the low a path so that the effect sought, namely a decrease in the term in Equation (4), will be obtained.

Considering first a three electrode electron discharge device having effectively plane parallel electrode surfaces, if a portion of the control electrode or grid is displaced outwardly toward the anode, the amplification factor a for this portion is decreased and the plate current for the section may or may not increase, depending upon the sign of the rate of change of plate current with respect to the grid-cathode spacing. For the desired condition this rate of change, which may be expressed as 25 B should be negative.

For an electron discharge device having effectively plane parallel electrode surfaces, the plate current In, may be expressed as (EB+IJIEC)3/2 IB K (may (5) where K is a numeric constant and v is the anodecathode spacing, 13 is the grid-cathode spacing and the remaining characters have the same significance as in Equation (3).

If M is considered as proportional to v[:, then LIB: -11? l o 2#(v26) (6) 5 B EB+MEC V+[.LB

In order that be negative to satisfy the desired conditions, as pointed out hereinbefore, the term which is always negative, must be less than the term which is practically always positive.

It will be noted from Equation (6) that if 1 is made some factor greater than unity times [3, that is 11:05,

where In order to obtain the desired conditions, therefore, it is necessary that 1 should be small and the ratio of anode-cathode and grid-cathode spacing should be large. The value of J is effective in defining the power output for a given percentage of second harmonic. output, assuming no a modulation is present. Forsmall values of f the power output is small and the magnitude of the second harmonic is relatively low.

The factors involved in the proper correlation of the grid and anode in electron discharge devices to obtain the desired conditions noted hereinbefore will be understood from the following:

The plate current in a three electrode discharge device may be considered as a function of the grid-anode spacing, at, and the gridcathode Assuming a to be proportional to on, and differentiating Equation (9) with respect to ,6

Q13 Q (l 3/2HEC' (10) B a+6+ufl B B'H- c +fl+u5 For a fixed value of on independent of changes in {3,

and a is constant, so that Equation (10) becomes From Equation (11) it will be apparent that for the conditions specified the latter condition as pointed out under the discussion of Equation (6) may be either positive or negative.

is always negative.

Since ach 5 E p s it follows that for some values of between 0 and 1, both a and In will be decreasing with increasing [5' and a grid section in a path of low wand high impedance is obtained as desired. Thus, it is possible by the proper design of the anode and grid to maintain negative even for those conditions, e. g. a large negative grid swing, when the term where Rp, Rg, and Re are respectively the radii of the anode, grid and cathode.

On difierentiating Equation (12) I KR,

13 p+!"( u c) c For devices of the type under consideration,

. 2 RF 21rnR 1og (14) H log coth21rn'y where n is the number of convolutions per inch in the grid and 'y is the radius of the grid Wire.

Difierentiating i=i 1R 6R R log fi;

It will be seen from Equation that if is made equal to e, or approximately 2.72,

will be zero and that for values of u 0 less than 2.72,

in 6R, will be negative. If

in (SR,

in Equation (13) is zero, it will be noted that in 5R,

will be negative and for relatively small negative values of the negative coeflicient of in ER,

will not be sufliciently large to make change sign. It follows, therefore, that an increase in the radii of sections of a control electrode or grid in an electron discharge device having cylindrical electrodes will provide the desired low a, low plate current sections.

Various forms of grids to provide the desired low [L high impedance sections may be employed. For example, as shown in Fig. 3, the grid G may be provided with a plurality of corrugations C, and the cathode or filament F and the anode P may have effectively plane parallel surfaces or concentric cylindrical surfaces. This construction provides a plurality of alternately arranged high and low a grid sections, the high ,u. sections having a low impedance and the low p. sections having a relatively high impedance.

In other embodiments illustrated diagrammatically in Figs. 5 and 6 the grids or control electrodes G may be concave or convex respectively with respect to the cathode or filament F.

A further reduction in second order modulation may be obtained by suitably varying the spacing between portions of the anode and the cathode. Thus, as indicated in Fig. 4 the surface of the anode may be provided with undulations or corrugations L in alignment with the corrugations in the grid surface, the depth of the corrugations L being less than that of the corrugations C and the corrugations being so formed that, as indicated in Fig. 4, the distance a between juxtaposed portions of the anode and the grid is greater than the distance b between other juxtaposed portions.

In other embodiments illustrated diagrammatically in Figs. 7 and 8 the anodes P are curved in the same direction as the corresponding grids G but to a lesser degree so that, as indicated in these figures, the distance a is greater than the distance b.

The electron discharge device shown in Fig. 1 is contained in an enclosing vessel l suitably secured to a base ll carrying a plurality of terminal prongs l2. The vessel It is provided with a reentrant stem portion 13 which terminates in a substantially rectangular press 14. A rigid metallic rod I is embedded in the press [4 and supports a heater type cathode having a cylindrical metallic sleeve [6 coated with an electron emitting material, such as barium and strontium oxides. The metallic sleeve I6 is electrically connected to one of the terminals [2 through a leading-in conductor l1 embedded in the press l4, and a wire strip l8 connected to the sleeve l6 and to the conductor H. The heater current for the cathode may be supplied through leading-in conductors l9, one of which is connected to the rod [5 and the other of which is connected to one end of the heater unit, not shown, by a wire 20.

A two piece flanged metallic collar 2! is clamped about the stem l3 and has secured thereto a plurality of bent metallic supports 22 which extend lengthwise of the vessel 10. A hollow anode 23 is provided with outwardly extending fianges 24 which are secured, as by welding, to the supports 22. Electrical connection is established between the anode 23 and one of the terminal prongs 12 through the intermediary of a conductor 25 sealed in a wall of the stem I3 and connected to a metallic strip 25 which is secured, as by welding to the collar 2!. An annular support 2'! is mounted on the strip 26 and carries a getter material, such as magnesium, for fixing residual gases within the enclosing vessel ID.

A plurality of metallic stubs 28 is secured to the supports 22 at the end thereof remote from the stem 13 and carries an insulating disc 29. A plurality of upright wires 30 is supported at one end by rigid short wires 3| embedded in the press l4 and the wires 3!] are seated at the other end in slots 32 in the insulating disc 29. A helical wire control electrode or grid 33 is supported on the wires 30 and is symmetrically positioned with respect to the cathode I6 and coaxial therewith, electrical connection between the grid 33 and one of the terminals prongs [2 being established through the intermediary of one of the rigid Wires 3|.

As shown more clearly in Fig. 2, the control electrode or grid 33 and the anode 23 are both oval or elliptical in lateral cross-section and are coaxially arranged with respect to the cathode Hi. The major axis of the oval defined by the grid wires is at an angle, preferably a right angle, to the major axis of the anode oval. This arrangement, it will be apparent, provides a plurality of electron paths of different a and different impedance between the cathode and the anode, the paths of high impedance having a low M and the paths of low impedance having a high ,u.

Although various forms for the control electrode or grid and for the anode have been shown and described, it will be understood, of course, that these forms are merely illustrative of the invention and that modifications may be made therein without departing from the scope and spirit of this invention as defined in the appended claims. It will be realized also, of course, that the invention is not limited to three electrode electron discharge devices but may be embodied in such devices having more than one grid, for example screen grid discharge devices. In devices of the latter type either or both the control electrode and the screen grid might be of any of the forms shown for the control electrode in the drawing.

What is claimed is:

1. An electron discharge device comprising a cathode, an oval shaped anode encompassing said cathode, and an oval shaped control electrode intermediate said cathode and said anode, said anode and control electrode having their similar lateral axes intersecting.

2. An electron discharge device comprising a rectilinear cathode, an elliptical shaped anode symmetrically disposed about said cathode, and an elliptical shaped grid intermediate said cathode and said anode and symmetrically disposed about said cathode, said anode and grid having their similar lateral axes intersecting at substantially right angles.

JAMES O. McNALLY. 

